Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175–179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States,” Phys. Rev. Lett. 68 3121 (1992) (hereinafter, “the Bennett 1992 paper”). The general process for performing QKD is also described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27–33.
The above mentioned publications and patent each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons, and Bob randomly measures the polarization or phase of the photons. The one-way system described in the Bennett 1992 paper and in the '410 patent is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
FIG. 1 is a schematic diagram of a two-way QKD system 10 that includes a conventional fiber-optic-based QKD station BOB, as disclosed in FIG. 4 of the article by Bethune and Park, “Autocompensating quantum cryptography,” New Journal of Physics 4, (2002) 42.1–42.15 (hereinafter, “the Bethune Article”) which Article is incorporated by reference herein. QKD transmitter BOB serves as a transmitter and receiver and includes a distributed feedback (DFB) laser 12, a variable optical attenuator (VOA) 14, a polarization controller 16 and a circulator 18, coupled in series via sections of optical fiber 20.
One port of circulator 18 is coupled via an optical fiber section 21 to a polarization-maintaining (PM) variable coupler 26. One port of the PM variable coupler 26 is coupled to an optical fiber section 22A that in turn is coupled to a coupler 30. Another port of coupler 26 is coupled to another optical fiber section 22B that includes a phase modulator 34. Optical fiber section 22B is also coupled to coupler 30. A third port of coupler 26 is coupled to an optical fiber section 40 that leads to a first single-photon detector (SPD) D1. Also, one of the ports of circulator 18 is coupled to an optical fiber 42 that leads to a second SPD D2. SPDs D1 and D2 are coupled to a controller 50. Controller 50 is also coupled to phase modulator 34.
In operation, light pulses P0 are emitted by laser 12 and attenuated by VOA 14. The attenuated light pulses are then polarized by polarization controller 16. Circulator 18 passes the pulses to PM variable coupler 26. At PM variable coupler 26, each light pulse is split into two light pulses PA and PB having different polarizations, with one light pulse (say, PA) directed to optical fiber section 22A, while the other light pulse (PB) is directed to optical fiber section 22B. Because pulses PA and PB are outgoing, pulse PB remains unmodulated by phase modulator 34. These pulses are then re-introduced into optical fiber channel 60 at coupler 30 with a relative time delay.
Pulses PA and PB travel over fiber channel 60 to a second QKD station ALICE, where one of the pulses (say, PB) is randomly phase-modulated by a second phase-modulator 70 after reflecting from a Faraday mirror 72, which rotates the polarizations of the pulses by 90°. Pulses PA and PB then travel back to BOB over fiber channel 60. At coupler 30 pulse PA is directed into fiber section 22B, where it is randomly phase modulated by phase modulator 34 via the operation of controller 50. Because pulse PA now is time-delayed by the same amount as pulse PB, it combines with pulse PB at PM variable coupler 26, where the pulses interfere with one another. Depending on the relative phase imparted to the pulses, the resulting combined pulse will either travel over optical fiber section 40 to SPD D1 or over optical fiber section 42 to SPD D2. The detection events are then counted as clicks in controller 50. These clicks are then processed using known techniques (e.g., sifting, error correction and privacy amplification), to create a secret quantum key shared by BOB and ALICE.
The fiber-based optical system of BOB has a number of significant drawbacks. First, there are a large number of optical fiber splices, which results in losses in the system. Second, system is not particularly compact because of the lengths of optical fibers needed to connect the various components. Third, the extinction ratio, while good, is difficult to improve in the optical fiber-based configuration.
While the Bethune Article also offers a bulk-optics configuration as shown in FIG. 1 therein, it requires six elements including Faraday rotators and waveplates, and does not include certain elements that should be included in a bulk-optics embodiment of BOB1's optic's layer to be used in a commercially viable QKD system.